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Jörg Sprave's day job is as a manager in the world of consumer electronics. But he has been for many years making manifest the sort of things that once filled my school notebook margins with doodles: slingshots and other devices for launching bolts, steel balls, and other stuff at high speed at targets or just into the air. (Some of his "slingshots" are hard to recognize as such; he eschews the classic American wrist-rocket braced design as well as the old Tom Sawyer forked branch in favor of things a bitmore elaborate.) Thanks to the Internet, hobbies that were once obscure are now easy to follow, and Sprave's homemade slingshots are no exception; you can follow his exploits through an ongoing series of YouTube videos and a forum site that builds on these videos. He's doing it in Germany, too, where firearms may be harder to come by than in the U.S., but giant honkin' firecrackers are available (at least for part of the year), and acts accordingly. Amazingly, he has yet to lose an eye; his goggles are a wise precaution. Sprave has agreed to answer your questions about his own take on physics as a hobby. As usual for Slashdot interviews, you're invited to ask as many questions as you'd like, but please divide them, one question per post.

From watching a few of your videos, I see a lot of end results which are fun to watch. Take, for example, the firecracker slingshot mentioned in the summary [slashdot.org]. But one thing I can't seem to find is any testing videos or explanation of testing (maybe you have it in German and I can't find it). So how much testing do you do on these? When I saw your firecracker launcher design I immediately thought that the worst case scenario is that the firecracker detonates before leaving the chamber or possibly barrel. Now, a firecracker that goes off in the palm of your open hand versus one that goes off with your fist wrapped around it produces two very different results. A short or dry fuse in your device could be problematic. Did you detonate firecrackers inside the chamber from a distance to see if there was enough room in the chamber to allow the explosion to vent without splintering your device? How much testing did you do on it before you were confident enough to pick it up and use the butane lighter on it? Is this discussed somewhere?

A normal spacecraft takes a few minutes, and a few thousand kilometers, to reach orbital velocity. If you try to reach the same velocity right at launch, you'd crush whatever is in the spacecraft. Then there's atmospheric friction.

So I see two possibilities:

1. A slingshot (or cannon, rail-gun, etc) to provide an initial push, then use a rocket. More complicated than what they're using now, but you'd need less rocket fuel, which could be a big deal.

I see the majority of your projects depend on Thera Band Gold and then the throwers (like your circular saw blade launcher) use a spring. Have you investigated other methods of using elastic energy (which I guess is technically potential energy?) to propel a an object forward by transferring to kinetic energy? Are you engaged in any areas of materials science study in order to find more efficient and reliable materials? Or do you just use Thera Band Gold as your gold standard and leave it at that?

I/my friends did crazy elaborate things in high school too... too bad there was no YouTube back then (or much of an internet), though I think I may have some 8mm movies at my parents' house. I mostly stuck with explosives and smoke bombs, but one of my friends built a portable rail gun that put a 6" nail through a 2x4 board (the 2" side) and fully embedded it into a cinder block (at least a couple of inches deep), and that wasn't even at full power. In fact, the AP physics teacher nixed a full power test, b

What legal issues have you encountered? If your authorities are anything like they are in the more "progressive" states of the USA, they consider anything that looks like a firearm or throws any solid object (or not so solid [cnn.com]), via any means of propulsion, a dangerous weapon.

With the recent controversy regarding assault weapons in the US and the plans to reduce clip sizes, I'm wondering if you would think your, um, devices would be more dangerous if they could shoot multiple objects rather than just one at a time?

This is at least the 2nd time I've seen this guy show up on slashdot. While I'm all for people building stuff in their garage (I do it for sure) I see nothing impressive in this gentlemans designs. Nothing new or interesting. I built something very similar to his nailgun when I was in 7th grade and found a box of large nails at a construction site. His hobby is great, but he needs to come up with something far more insightful before I consider it news worthy. Why is this making it on slashdot?

Low density is the key. I suppose I have to officially discourage this kind of activity, but in the early 90s my friends and I had indoor potato gun fights using balls of crumpled paper as ammo. Virtually harmless projectiles even at point blank range assuming you use gas fuel and not liquid fuel. Liquid fuel sets the projectile on fire, which sounds cool but usually isn't unless you're on dueling pontoon boats pretending you're re-enacting naval battles, which is a whole nother story. One broken glass

*Why* is so much of our science dangerous? Why not *marry* safe science if you love it so much? In fact, why not invent a special safety door that won't hit you in the butt on the way out, because *you are fired!*

William(who made a crossbow using a truck leaf spring when he was a teen-ager and used his father's ratchet lever hoist to cock it --- was fortunate to be to on the opposite side of the tree when the stock split and things went flying)

The last step for Wendel was 'get arrested by airmen in helicopter while putting out scattered fires, deny anything to do with ultimate water rocket, just good citizens abating a hazard. Get away with it.'